This invention relates to restoring performance of PEM fuel cells after performance loss due, for instance, to freeze/thaw cycles and/or xe2x80x9cboot strapxe2x80x9d startup of frozen fuel cells.
In U.S. Pat. No. 4,294,892, the regeneration or recovery of a fuel cell (restoring the fuel cell to full performance following a loss of performance) is accomplished by reducing the concentration of oxygen on the cathode while maintaining the overall pressure of gas in the cathode oxidant reactant flow fields, either by replacing oxygen with a flow of argon or by shutting off the air discharge. The terminal voltage is preferably reduced to 10% of its original value. Other known methods of regenerating a fuel cell include reducing or stopping the flow of air and substituting a flow of inert gas for the oxidant flow. In most cases, fuel flow continues so as to react with and thereby remove oxygen from the system. In some cases, the load remains connected and in other cases an auxiliary load is substituted for the principal load.
A known method of regenerating the performance of a fuel cell called xe2x80x9chydrogen pumpingxe2x80x9d is disclosed in U.S. patent application Ser. No. 09/602,361, filed Jun. 22, 2000. The load is disconnected from the fuel cell stack and a reverse-polarity power supply is connected across the fuel cell stack output. Then, hydrogen is flowed through both the anode and cathode flow fields. This causes hydrogen ions, or protons, or hydronium ions to flow from the cathode to the anode, and reduces the cathode potential, typically, to below 0.1 volts. The flow of hydronium ions from the cathode to the anode transfers water from the cathode to the anode and is referred to as proton drag. The process is continued until it is known that the fuel cell stack will revert to normal performance when normal operation of the fuel cell stack resumes. In another embodiment, the air is replaced by inert gas with the fuel cell stack output being open circuited (no load). Increasing current may also be used to help reduce the voltage across the cells and to help regenerate performance.
The use of fuel cell power plants in vehicles require that they be operable in ambient temperatures as low as xe2x88x9230xc2x0 C. and that they be capable of startup in several minutes or less. One method of starting a frozen fuel cell is supplying hydrogen and air to the anodes and cathodes, respectively, and to immediately draw electricity from the fuel cell, with the normal fuel cell coolant being diverted around the fuel cell. This is called a xe2x80x9cboot strap startxe2x80x9d. The waste heat produced within the cell, as a by-product of the fuel cell reactions, causes the stack temperature to rise very quickly. However, repetitive use of this procedure has been found to reduce fuel cell stack performance in substantially the same fashion as repetitive freeze/thaw cycles.
Objects of the invention include restoration of performance to fuel cell stacks following loss of performance such as from repetitive freeze/thaw cycles or repetitive boot strap start-up cycles; a fuel cell stack regeneration process which can be monitored for effectiveness; and an improved process for restoring fuel cell stack performance.
It is known that the resistance of a proton exchange membrane, such as a perfluorosulfonic acid membrane frequently used in PEM fuel cells, increases as the water content of the membrane is reduced. This invention is predicated on the discovery that partially drying out the nano pores present in the proton exchange membrane, or in the ionomer of the catalyst layer adjacent to the membrane, will substantially restore fuel cell performance.
According to the present invention, the performance of a fuel cell stack is recovered by drying out the fuel cells to the point where the resistance across each cell has increased significantly, such as at least by 5 to 1, and preferably by 10 to 1, over the normal resistance of the cell. In accordance with the invention, water is evaporated from all of the macro pores (pores that are greater than about 100-200 nanometers in diameter or larger) in the cell structure, including pores in the water transport plates if present, the substrates, the diffusion layers, and the catalyst layers. With all of the water evaporated from the macro pores, the micro pores present in the proton exchange membrane or in the ionomer within the catalyst layer (which may be approximately four nanometers in diameter) are partially dried out. Completion of dry out is determined by measuring the resistance of the fuel cells. The resistance can be measured across each cell individually, or, in an otherwise properly constructed fuel cell stack in good condition, resistance can be measured collectively, across the entire fuel cell stack.
In accordance with the present invention, a water absorbing gas is flowed into at least one of: (1) the fuel reactant gas flow fields, (2) the oxidant reactant flow fields, and the (3) water management flow fields (if any) of all of the fuel cells. In some cell constructions, the water flow fields also function as coolant flow fields. The water absorbing gas may be ambient air, nitrogen or other inert gas, or any other gas which absorbs water suitably and is otherwise not adverse to the conditions and process involved. The gas may be at ambient or elevated temperature. The gas may be at ambient pressure or pressurized.
According further to the present invention, water is removed from a fuel cell by evacuating at least one of: (1) the fuel reactant gas flow fields, (2) the oxidant reactant flow fields, and (3) the water management flow fields (if any) of all the fuel cells. In accordance further with the present invention, one or more of the flow fields may be evacuated contemporaneously with providing a water absorbing gas to such flow fields.
The present invention not only provides processes which restore fuel cell performance lost either as a result of repetitive freezing or repetitive use of boot strap start-ups, but it does so in a way that can be monitored by means of resistance so that it is known when the process is complete and will be successful.
Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.